Poor fish. He does sound like he's on "his last leg." Have you checked the goldfish forum on here? They may be able to help you more since they are familiar with goldies.
Have you tested your tap water before treating for the ammonia and nitrites? You could be right of the tap water having trace amounts. I've heard of that. There are certain products that make them less toxic like Ammo-Lock for the ammonia. And Amquel for the nitrites. At least I think that's what it's called. I've never used either. There are old threads around here with discussion on these products but you'd have to do some digging around.
Pimafix is stronger than Melafix. In more serious cases of bacterial problems I suggest Pima. I don't know about mixing it with Quick Cure though. I've never done that. I think Mela is too mild unless used at the very first signs of problems. It's not gonna help truly aggressive fin rot. At least it hasn't in my case.
ok, I just done a quick google on Amquel. Sounds like Amquel+ would be good for you. Check this out:
Kordon LLC - Kordon - AmQuel+